The invention relates generally to stent delivery systems including catheters similar to the kind used, for example, in percutaneous transluminal coronary angioplasty (xe2x80x9cPTCAxe2x80x9d) procedures. More particularly, the invention pertains to the use of a compliant membrane for retaining the stent on the catheter during delivery through the patient""s vascular system.
In typical PTCA procedures, a guiding catheter is percutaneously introduced into the cardiovascular system of a patient through the brachial or femoral arteries and advanced therethrough until the distal end thereof is in the ostium of the desired coronary artery. A guide wire and a dilatation catheter having a balloon on the distal end thereof are introduced through the guiding catheter with the guide wire sliding within the dilatation catheter. The guide wire is first advanced out of the guiding catheter into the patient""s coronary vasculature and the dilatation catheter is advanced over the previously advanced guide wire until the dilatation balloon is properly positioned across the lesion. Once in position across the lesion, the flexible, expandable, preformed balloon is inflated to a predetermined size with radiopaque liquid at relatively high pressures (e.g., greater than about 4 atmospheres) to radially compress the atherosclerotic plaque of the lesion against the inside of the artery wall and thereby dilate the lumen of the artery. The balloon is then deflated to a small profile, so that the dilatation catheter can be withdrawn from the patient""s vasculature and blood flow resumed through the dilated artery.
In angioplasty procedures of the kind referenced above, there may be restenosis of the artery, which either necessitates another angioplasty procedure, a surgical bypass operation, or some method of repairing or strengthening the area. To reduce the likelihood of the development of restenosis and strengthen the area, a physician can implant an intravascular prosthesis for maintaining vascular patency, called a stent, inside the artery at the lesion. The stent is expanded to a larger diameter, often by the balloon portion of the catheter. Stents delivered to a restricted coronary artery, expanded to a larger diameter by a balloon catheter, and left in place in the artery at the site of a dilated lesion are well known in the art as are various delivery systems used to deliver and implant a stent. Typical stent delivery systems can be found in U.S. Pat. No. 5,514,154 (Lau et al.).
A shortcoming of previously known stent delivery systems is the possibility of the stent shifting off the balloon while being maneuvered through a patient""s vasculature. Contact with plaque or other obstructions encountered within the vasculature pose a risk of impeding the progress of the stent while the underlying balloon and catheter continue to be advanced. Stent shifting on the balloon or stent loss also exists when trying to retract the stent into the guiding catheter in the event the stent could not be delivered.
In one prior art device stent shifting or stent loss has been addressed with the use of an elastomeric protective membrane that is disposed about the balloon and attached to the catheter. In addition to controlling the shape of the balloon, protecting the balloon and containing any debris should the balloon rupture, the membrane provides a pliable surface into which the unexpanded stent can be crimped. This results in an enhanced grip that more effectively prevents the stent from being prematurely dislodged from about the balloon prior to inflation.
Disadvantages associated with such approach include the potential for the membrane to interfere with the performance of the balloon. Because the membrane is located between the balloon and stent, the stent conceivably may become so imbedded in the membrane during inflation of the balloon by virtue of the substantial pressure exerted by the balloon against the stent such that stent fails to completely detach from the membrane upon deflation of the balloon. This may prevent the retraction of the delivery system after the stent has been irreversibly expanded against the vessel walls. Additionally, the surface texture of the compliant membrane may also generate undesirable friction between the membrane and the stent as the delivery system is being withdrawn and between the membrane and the vessel walls as it is withdrawn therethrough.
Other prior art catheter systems have shoulders or rings on the balloon at either end of the stent to help hold the stent in place, but such devices have a high profile. A protective retractable sheath over the balloon and stent is also known in the art but such systems have a high profile and are much less flexible and therefore are more difficult to deliver in the patient""s vasculature.
An improved stent delivery system is needed that takes full advantage of the balloon""s low friction surface, allows a stent to firmly grip the delivery balloon catheter, positively ensures stent release after deployment and deflation of the balloon, provides a low profile, and ensures flexibility of distal portion of the stent delivery catheter.
The present invention is directed to the use of an elastic or pliable membrane that provides a compressible mass into which a stent can be crimped to ensure its retention about a delivery catheter and balloon. The membrane is positioned within the expandable balloon in the form of a tubular sheath affixed about the underlying catheter.
The compliant membrane into which the struts of the stent are crimped provide substantially more friction to retain the stent than is achieved by the extremely thin and virtually frictionless surface of the balloon. Additionally, because the membrane is positioned within the balloon, it does not in any way participate in nor can it interfere with the deployment of the stent. In fact, during inflation of the balloon, the balloon serves to effectively separate the stent from the membrane. This ensures that the delivery system can easily be withdrawn from the vasculature. Moreover, the slippery nature of the outer surface of the balloon can be taken full advantage of when withdrawing the catheter delivery system after deployment of the stent.
In one preferred embodiment, a delivery catheter has an expandable balloon adjacent its distal end. The catheter preferably has an inner member and an outer member at least in the region of the distal portion of the catheter, the inner and outer members having a coaxial relationship. An elastic membrane positioned on the inner member in the area of the balloon so that the stent, which is mounted and crimped on the balloon, is positioned over the elastic membrane. Preferably, the membrane is thin, highly elastic, and has a diameter smaller than the inner member so that it tightly grips the inner member and will not shift. While it is preferred that the membrane has a uniform thickness, the ends of the membrane that extend beyond the stent can be thicker thereby forming a ridge on either end of the stent to protect the ends and prevent stent shifting or stent loss. As the stent is crimped tightly onto the balloon, the elastic membrane tries to fill the spaces between the stent struts, thereby providing a gripping action on the stent.
A number of different membrane configurations may be employed to practice the present invention including embodiments wherein the membrane is shorter, longer, or of the same length as the stent. Each such embodiment provides certain advantages relating to the amount of gripping force afforded thereby and to the smoothness of the profile of the resulting assembly. Further, the membrane may have proximal, distal and center sections that align with the stent to provide the required gripping force. The membrane may have a radiopaque material impregnated in or coated on it to more easily identify the position of the stent in the vasculature.